As is known, the care of serious damage to a synovial joint resulting from accidents is challenging. For example, falling accidents often result in serious damage to the ankle, which is caused by the ankle bone impacting the cartilage surface of the tibia, which in the worst case can even lead to the crushing of the lower end of the tibia. Recovery from injuries like those described usually takes several months. In typical care following a falling accident, the damaged ankle is repaired operatively and fixed, i.e. supported rigidly, using, for example, so-called pilon rings and similar care accessories. However, in order to recover to full functionality, the cartilage requires nutrition, the transportation of which—unlike that in other tissues—is based on the tissue being loaded in cycles, so that fluid dynamics appear inside the cartilage. The recovery of cartilage is described in detail in the publication, ‘Influence of cyclic loading on the nutrition of articular cartilage’ (O'Hara B., Urban J., & Maroudas A., Ann. Rheum. Dis. 1990 July; 49(7): 536-539). If mobilization that transports nutrients is not arranged, the cartilage surface repaired by the operation may be destroyed, which will be followed in a couple of years by a state corresponding to osteoarthritis, i.e. invalidity. Precisely because osteoarthritis patients are mostly young people or those of working age, such as building workers, invalidizing osteoarthritis leads to not only personal misfortune, but also a significant economic cost.
In the publication ‘Articulated external fixation of the ankle: minimizing motion resistance by accurate axis alignment’ (Bottas M., March. L., & Brown T., Journal of Micromechanics, Vol. 32, No. 1, January 1999, pp. 63-70), it is stated that factors promoting recovery from, for example, the ankle-fracture injuries referred to above are protection from loads, early post-operative movement, a reduction in splinter fractures, and minimal disturbance of the injured area. For this reason, post-operative supports for damaged joints have been developed, so that in aftercare it will be possible to take into account mobilization of the joint as a precondition for recovery. However, it should be noted that, besides the mobilization of a damaged joint, its correct timing is of considerable significance in the success of rehabilitation. For example, the mobilization of an ankle must be started already two days after an operation. Correspondingly, a movement of the wrong kind can have disadvantageous consequences. It is therefore of decisive importance to find the joint's anatomically correct path, in order to minimize the resistance to motion and avoid sudden damage caused by the wrong kind of movement. Thus, significant expectations are directed to post-operative supports, in relation to both being able to be rapidly installed and to creating the correct type of path.
Many external supports are known. However, the majority of supports intended for the aftercare of synovial joint injuries are either rigid, i.e. the supports do not permit therapeutic movement, or supports permitting movement, the motion permitted by which is typically a rough approximation of the real movement of the joint. In a hinge joint, such the ankle, movement takes place around only a single axis of rotation, with a limited extent of movement. This is the simplest model of a moving joint, due to which it is used as an illustrative example in this connection. In other types of synovial joint, rotation and sliding in the direction of several axes or planes of movement can take place simultaneously. These can be controlled equally by means of the technology disclosed here. Rigid supports are, among others, Ilizahrov rings, which are external supports attached on both sides of the damaged joint. Ilizahrov rings are a way of implementing joint support that penetrates the tissue, i.e. it is invasive. In the method, the rings are attached to the patient's bone by using tensioning cables and bone screws. Ilizahrov rings and their use are described in greater detail in the publications ‘Pilon fractures. Treatment protocol based on severity of soft tissue injury’ (Watson J. T., Moed B. R., Karges D. E., Cramer K. E. Clin. Orthop. 2000; 375: 78-90) and ‘Two-ring hybrid external fixation of distal tibial fractures: A review of 47 cases’ (Ristiniemi J., Flinkkilä T., Hyvönen P., Lakovaara M., Pakarinen H., Biancari F., Jalovaara P., J. Trauma 2007; 62: 174-183), the contents of which is included in this as a reference. In addition, non-invasive rigid supports are known, such as traditional plaster casts and similar. Supports permitting movement have been created, for example, by arranged external hinge-type plates, with the aid of which an attempt has been made to imitate the movement of the damaged joint. An example of the said plate in cases like the ankle fracture described above is a kind of pedal, on top of which the base of the foot is placed and which is adjusted to permit only such a tilting movement as would be natural for a healthy ankle.
Alternative methods are known for defining the natural movement of a synovial joint. In camera-based methods, the movement is recorded by using, for example, a video camera and alignment marks, which are attached to the object to be moved. After recording the movement, the preferably digital video material is analysed using special software and the movement information obtained with the aid of the alignment marks is captured, in order to form the path of movement. This method is utilized widely, for example, in sports applications and in the film industry, for which the technology was originally developed. Because the method does not require physical contact with the patient, the method is quite user-friendly from the patient's point of view. The accuracy of the method varies from the accuracy required for making animations to the accuracy required for quality control. However, in the final resort the accuracy of the method depends on the resolution of the camera and on the measurement volume used. Typically, sufficiently accurate information is obtained by means of the method for animation of the movement of an entire limb, but this technology does not provide an answer to the movements of the bones that act as counter-surfaces in an individual joint. A drawback of the method is that, in terms of the area of the theme of the invention, the method cannot be used to determine reliably the movement of the bones under the actual tissues, but rather the movement of the tissue on top of the bones. In addition, these methods do not reveal the fine-dynamic flexing under the soft tissue, i.e. the dynamics between the bones. Because it has not been possible to accurately define the precise anatomic movement, it has also not been possible, on the basis of these methods, to design anatomically personalized external supports.
An alternative to camera-based methods are three-dimensional or radiographic methods, in which a three-dimensional model of the bones is formed on the basis of either computer tomography (CT) or magnetic-resonance imaging (MRI). The methods are suitable for modelling the shape of an individual bone. MRI is not, however, suitable for situations in which non-MRI compliant screws are used, for example certain steel screws or other attachment means, in the area of the joint already attached for old injuries or installed for the care of a new injury. In the said cases, CT imaging would be a possible method, but it suffers from imaging interference caused by metals and from the great radiation stress caused to the patient.
In known applications, a damaged synovial joint and its part are modelled on the basis of CT or MRI, when a virtual kinetic model corresponding to the damaged joint is obtained. This solution has been typically used in early motion analysis studies of cases of injury, because the technology used has been readily available in a hospital environment. For example, publication US2008312659 discloses a method for manufacturing a prosthesis, in which a patient-specific image, which is used as an aid in the manufacture of the prosthesis, is formed from data obtained from MRI imaging. For its part, publication US2007118243 discloses a method, in which a computer-based model, which is exploited to manufacture implants, prostheses, and similar, is created from data obtained on the patient's anatomy in CT imaging. Though CT and MRI-based methods are indeed suitable for the manufacture of patient-specific artificial joints and other implants, the use of the said methods does not achieve sufficient accuracy as would permit preserving and saving a patient's own joint after injury. Traditionally, it has been possible to achieve an accuracy of about 10 millimeters, whereas achieving a good result would require an accuracy of at least 1 . . . 3 millimeters, preferably at least 0.5 millimeters. Typically, significant swelling also occurs in the area of a limb joint after injury, which reduces the accuracy if the definition of movement or the support is based on skin contact.
In general, significantly unknown tolerances relate to the technology used in the creation of bone models, which derive from the imaging quality and the grey-tome values available in sectioning. In addition, the joints, locations, and attitudes of three-dimensional models are fitted together visually in a 3D environment, which further reduces the method's reliability and repeatability. Tolerance errors made in the creation of bone models accumulate, when the attachment points are designed on the basis of the models. All in all, at least up until now, the CT and MRI-based three-dimensional method have not been applied, because sufficient accuracy cannot be achieved using the methods.
Thus, the problems of the prior art are related to the determining of the path of a damaged joint. Because each joint, tissue, and injury is different, a statistical approximation and present modelling methods have not been able to provide a solution for creating an anatomically personalized support. More specifically, using present post-operative external supports, i.e. supports external to the body, it has not been possible to place artificial or auxiliary joints sufficiently precisely on the paths of movement of the joint, so that the mobilization of an injured limb or similar will not succeed, due to which the cartilage of the joint will not receive nutrition reliably. As stated, in mechanical design, as is known, reference geometries can be utilized, either by creating them in a three-dimensional 3D-CAD system, or by bringing a camera-based digital geometry to the design system, by using various methods and various formats. Challenges generally arise in the combination of a reliable design geometry, referencing digitalization, and a real application. Thus, the known joint supports have been rigid, which is not optimal from the point of view of the recovery of a joint.
The external support devices on the market, which a priori permit movement to a limited extent around a single axis, are in point of departure universal-type devices. It has therefore not been possible to take into account the size of the patient or soft-tissue damage, which are important in terms of avoiding complications. In these cases, the attachment spikes must be placed in an area that has been very precisely defined beforehand, while the location of the external axis cannot be determined other than visually with the aid of transillumination. The precision then remains unavoidably poor and the path small.
It is an object of the present invention to solve at least some of the drawbacks of the prior art and to create an improved method for creating a anatomically personalized and mobilizing external support for rehabilitating a synovial joint.